linux-kernel - Re: Default enable RCU list lockdep debugging with PROVE

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Date:   Thu, 14 May 2020 09:39:45 -0400
From:   Qian Cai <cai@....pw>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        Amol Grover <frextrite@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: Default enable RCU list lockdep debugging with PROVE_RCU



> On May 14, 2020, at 9:33 AM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Thu, May 14, 2020 at 08:31:13AM -0400, Qian Cai wrote:
>> 
>> 
>>> On May 14, 2020, at 8:25 AM, Stephen Rothwell <sfr@...b.auug.org.au> wrote:
>>> 
>>> Hi Paul,
>>> 
>>> This patch in the rcu tree
>>> 
>>> d13fee049fa8 ("Default enable RCU list lockdep debugging with PROVE_RCU")
>>> 
>>> is causing whack-a-mole in the syzbot testing of linux-next.  Because
>>> they always do a debug build of linux-next, no testing is getting done. :-(
>>> 
>>> Can we find another way to find all the bugs that are being discovered
>>> (very slowly)?
>> 
>> Alternatively, could syzbot to use PROVE_RCU=n temporarily because it can’t keep up with it? I personally found PROVE_RCU_LIST=y is still useful for my linux-next testing, and don’t want to lose that coverage overnight.
> 
> The problem is that PROVE_RCU is exactly PROVE_LOCKING, and asking people
> to test without PROVE_LOCKING is a no-go in my opinion.  But of course
> on the other hand if there is no testing of RCU list lockdep debugging,
> those issues will never be found, let alone fixed.
> 
> One approach would be to do as Stephen asks (either remove d13fee049fa8
> or pull it out of -next) and have testers force-enable the RCU list
> lockdep debugging.
> 
> Would that work for you?

Yes, if there is a way to enable PROVE_RCU_LIST=y manually, that is fine. I think we would want to make it easier to enable it. Currently, it is buried into RCU_EXPERT?